1. Field of the Invention
The invention relates to the process of designing and fabricating semiconductor chips. More specifically, the invention relates to a method and an apparatus for improving depth of focus of a mask layout during optical lithography.
2. Related Art
Recent advances in integrated circuit technology have largely been accomplished by decreasing the feature size of circuit elements on a semiconductor chip. As the feature size of these circuit elements continues to decrease, circuit designers are being forced to rely on resolution enhancement techniques, such as assist features, to improve depth of focus during optical lithography.
Unfortunately, assist features cannot be used in certain situations. For example, certain illumination techniques, such as dipole illumination, impose strict restrictions on the pitch, which can make it impossible to find an appropriate location to place an assist feature. Similarly, it may not be possible to place assist features in fully nested line and space patterns.
Hence what is needed is a method and an apparatus to improve the depth of focus without the above mentioned problems.